Chemical mechanical polishing to achieve the planarization of substrate surfaces, such as those of semiconductor wafers, flat panel displays, hard disks, etc. has become a very desirable method of processing. CMP typically requires the mounting of a substrate into a head or carrier which is then urged against a polishing surface to effect polishing of an exposed surface of the substrate. In the usual arrangements, both the carrier and the polishing surface are rotated to apply a polishing action.
For example, Kaanta et al., U.S. Pat. No. 5,036,630, discloses a method of polishing a semiconductor wafer in which a wafer carrier is coupled to a spindle which is in turn driven by a motor to rotate the spindle and wafer carrier. The wafer carrier applies a load to the wafer and against a rotatable turntable assembly which includes a polishing table that is rotatably driven by a motor.
Hirose et al., U.S. Pat. No. 5,384,986, discloses a turntable with an abrasive cloth mounted thereon and a top ring, each of which are independently rotated to perform polishing. The top ring drive shaft is rotatable about its own axis by a train of gears which are rotated by a motor.
Sandhu et al., U.S. Pat. No. 5,486,129, discloses a rotatable platen assembly which is coupled to a drive mechanism for rotation thereof. A head assembly supports and holds a face of a semiconductor wafer in contact with the platen assembly to polish the wafer face. A motor is connected to the polishing head to rotate the polishing head. Individual regions of the wafer face are disclosed as having different polishing rates.
Shendon, U.S. Pat. Nos. 5,624,299 and 5,582,534, disclose a device for chemical mechanical polishing that includes a housing which is configured to provide orbital and rotational movement of a carrier. A gear arrangement is provided to rotationally drive the carrier while at the same time sweeping the carrier arm through an orbital path. A motor and gear assembly may be connected to a platen to provide a rotational polishing surface against which the carrier moves.
The ideal substrate polishing process can be described by Preston's equation: R=K.sub.p *P*V, where R is the removal rate; Kp is a function of consumables (abrasive pad roughness and elasticity, surface chemistry and abrasion effects, and contact area); P is the applied pressure between the wafer and the abrasive pad; and V is the relative velocity between the wafer and the abrasive pad. As a result, the ideal CMP process should have constant cutting velocity over the entire wafer surface, constant pressure between the abrasive pad and wafer, and constant abrasive pad roughness, elasticity, area and abrasion effects. In addition, control over the temperature and pH is critical and the direction of the relative pad/wafer velocity should be randomly distributed over the entire wafer surface.
Most of the current CMP machines, including those discussed above, fail to produce constant velocity distribution over the entire substrate surface and thereby fail to achieve uniform material removal over the entire surface which is essential for a planar result. Consequently, wastage of significant portions of the substrates results, particularly at the edges of the substrates.
Other relative motion arrangements have been attempted and described, but also fail to achieve constant velocity distribution over the entire substrate surface and thereby fail to achieve uniform material removal over the entire surface of the substrate.
Chisolm et al., U.S. Pat. No. 5,522,965, discloses a compact system for chemical mechanical polishing which employs a non-rotational platen having a polishing pad thereon, against which a wafer is rotated by a rotating carrier. An ultrasonic energy is inputted to the platen in an effort to enhance the polishing action.
Hirose et al., U.S. Pat. No. 5,643,056, discloses a revolving drum type polishing apparatus A rotating drum having a polishing pad mounted on its outer peripheral surface is provided and is rotationally driven by a motor about its longitudinal axis. The drum is suspended above a wafer to be polished by a column attached to a base. The wafer is seated on a Y-table which is in turn mounted on a X-table which is fixed to the base. The X and Y tables are able to oscillate in directions perpendicular to one another, while the drum rotates against the surface of the wafer.
Lund, U.S. Pat. No. 5,643,044, discloses an orbiting wafer carrier which is mechanically driven by an internal gear arrangement. An abrasive tape is forcibly pressed against an exposed surface of the wafer, during the orbiting motion to effect polishing.
Parker et al., U.S. Pat. No. 5,599,423, discloses an apparatus for simulating a chemical mechanical polishing system in an attempt to optimize the same. A rotating platen is provided, against which a polishing pad forces a substrate. The force is applied to the polishing pad by a moveable tubular polishing arm which is preferably continuously moved linearly across the rotating substrate, from edge to center, until the polishing end point is attained.
In addition to the failure to develop an apparatus which removes a consistent amount of material across the entire face of a substrate during polishing, most of the current machines discussed require a large mass to be born by the carrier or head support due to the mechanical arrangements which are provided for driving the carriers. This equates to a large inertial mass which must be contended with when starting and stopping a polishing motion. For rotational carriers, this is not a significant concern unless the rotational speed is to be frequently varied. However, rotational carriers have the inherent drawback of not providing a constant velocity distribution across the polishing surface.
Co-pending U.S. application Ser. No. 08/443,956, entitled "Method and Apparatus for Chemical Mechanical Polishing, discloses apparatuses which are capable of polishing a substrate while maintaining uniform average velocity between the substrate and an abrasive pad against which the substrate is polished. U.S. application Ser. No. 08/443,956 is hereby incorporated by reference thereto in its entirety.
For example, one embodiment disclosed in application Ser. No. 08/443,956 includes a carrier which is driven in the Z-direction by a servo motor and lead screw. A cross member, post and linear slide must be supported during programmable movements by the servo motor and lead screw. The carrier is maintained substantially fixed in the X and Y directions during polishing. A table, which includes the polishing surface against which the carrier polishes the substrate, is moved in the X and Y directions during polishing. The table is mounted along a linear slide and is moveable therealong in the X direction by a lead screw and servo motor arrangement. For movement in the Y-direction, a plate is provided which supports the table and is in turn mounted to another slide for movement therealong in the Y direction. The plate is driven by a third servo motor and lead screw arrangement.
While the above discussed embodiment, as well as the other embodiments disclosed in the application, effectively maintain uniform average velocity between the substrate and the abrasive pad during polishing, they nevertheless require the movements of fairly significant inertial masses to accomplish their functions. For example, in the embodiment described above, the Y-direction servo motor and lead screw must drive the combined weight of the plate and a portion of its slide, as well as the table, the table slide and the servo motor and lead screw associated with the X-direction movement of the table. This puts a significant strain on the servo motors, particularly the Y-direction servo in this example, which could lead to overheating and reduced service life of the servo and or lead screw. Even more significantly, the substantial masses involved limit the effective velocities at which the polishing patterns can be carried out.
Thus, there remains a need for systems with improved polishing velocity capabilities, which can at the same time maintain uniform average velocity between a substrate and an abrasive pad against which the substrate is polished. An important objective is to reduce the inertial mass or masses to be moved, especially for devices that include starting and stopping motions or variations in patterns and/or velocities during their operation. Another goal is to improve the performance of the drivers which actually move the inertial masses through their polishing patterns. More responsive drivers, i.e., drivers with improved acceleration and velocity capabilities, are desirable.
Additionally, mechanical arrangements for driving a carrier can limit the size of the polishing pattern that the apparatus is capable of performing. For example, the radius of the polishing path of the apparatus described in U.S. Pat. No. 5,643,053, is limited to the distance between the drive shaft 56 and the second shaft 64 which interconnect the carrier with a motor. It would be desirable to have a capability to define a polishing pattern which would be limited only by the useable surface of the polishing surface against which the carrier travels.